Catch up to FreeBSD VM object read/write locks

Upstream r248084 changed the vm_object mutex to be a rwlock,
allowing for future optimizations.  This is a KPI change, so
introduce conditionals to be compatible with both versions of the KPI.

Change-Id: I6e3101bc80262480035dee4c5b2d1b9cbc44b57b
Reviewed-on: http://gerrit.openafs.org/10295
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>